Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of Si) lower frequency applications. These more familiar semiconductor materials may not be well suited for high power and/or high frequency applications, however, because of their relatively small bandgaps (e.g., 1.12 electron volts (eV) for Si and 1.42 eV for GaAs at room temperature) and/or relatively small breakdown voltages.
In light of the difficulties presented by Si and GaAs, interest in high power, high temperature and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials, typically, have higher electric field breakdown strengths and higher electron saturation velocities as compared to GaAs and Si.
In Group III nitride semiconductor devices, as with semiconductor devices fabricated in other material systems, low-resistance, stable ohmic contacts are critical for the performance and reliability of the semiconductor devices. Thus, there is a need for improved ohmic contact structures and methods of fabrication thereof for Group III nitride semiconductor devices.